Cancer susceptibility is a complex genetic trait caused by an accumulation of defects in combinations of genes. Our laboratory is involved in projects designed to identify, characterize and clone tumor susceptibility/resistance genes in mouse B cell (plasmacytoma) cancer model systems. In the mouse plasmacytoma system, we have documented that at least 4 genes are involved in determining the susceptibility patterns seen in BALB/cAn mice. Three of these genes, Pctr1-3, reside in the mid and distal portions of mouse Chr4. The Pctr1 region shares linkage homology with human 9p21 and 1p31. The Pctr2 and 3 regions in the mouse also share linkage homology with human Chr 1p, abnormalities of which have been documented in a variety of human cancers, including breast, neuroblastoma, melanoma, Burkitt's lymphoma and multiple myeloma. The cyclin dependent kinase inhibitor Cdkn2a (p16, p19) was mapped within the Pctr1 critical region. Two sequence variants were identified between susceptible and resistant mice. Site-directed mutagenesis experiments of Cdkn2a were performed to construct GST-fusion proteins containing the allelic variants associated with the susceptible strain. These proteins did not inhibit phosphorylation of RB in kinase assays and did not bind to CDK4-CyclinD2 complexes. Furthermore, when the DBA/2 p16 variant was transfected into two separate plasmacytoma cell lines, many cells were blocked at G1. In contrast, cells transfected with the BALB/c variants continue to cycle. Plasmacytomagenesis is accelerated in knock-out mice. Furthermore, BALB/c p16 fails to suppress ras-transformation of NIH3T3 cells. DBA p16, as well as, both BALB and DBA p16 and p19 are capable of suppressing ras transformation. These data strongly implicate the Cdkn2a locus as the Pctr1 tumor susceptibility gene. Genetic variation has also been found in the p16 promoter and differences in promoter activity have been associated with one of the variants. Recent work has documented that the ras related element binding protein, RREB, binds preferentially to the BALB promoter and acts to repress p16 expression. In the Pctr2 region, a series of tumor induction studies coupled with allelotypic analyses of a series of recombinant congenic strains have narrowed the genetic interval harboring the susceptibility/resistance gene to 1-2 cM. The Pctr2 locus was found to act in a codominant fashion in studies of F1 hybrids between resistant and susceptible congenic strains of mice. Phenotypic effects of the Pctr2 locus were evident in delaying the onset of tumorigenesis and reducing tumor incidences. Efforts have been concentrated on developing a physical map of the region. Sample sequencing and EST matches have identified at least 11 new genes in the Pctr2 intrerval. These genes are being evaluated as candidates for the Pctr2 locus. To date, one of the genes (Frap, mTOR) differs in amino acid sequence between BALB/c and DBA. Functional assays have demonstrated a defect in kinase activity associated with the susceptible allele. Both p53 and phas1 are downstream substrates of FRAP kinase activity and for both substrates, BALB/c FRAP did not phosphorylate the proteins as well as DBA/2 FRAP. Additional studies have shown that BALB/c FRAP does not suppress ras transformation as effectively as DBA/2 FRAP.